The present invention is generally related to mass media drive controllers, including HDD drives, and more particularly to a write drive control circuit controlling the head of the drive.
The coil current waveform characteristics that are important in a HDD voltage write driver circuit is depicted at 10 in FIG. 1. In particular, the current rise time, overshoot amount and duration, undershoot, and settling time are of particular interest. Desired characteristics for the coil current waveform is a fast rise time and settling time, a controllable amounts of overshoot, and very little undershoot. By achieving these characteristics, a hard disk drive storage capacity can be improved.
A conventional voltage source based write coil driver circuit is shown at 20 in FIG. 2. A coil is modeled as LS, and is connected to four transistors via coil node HX and node HY. The connection of the four transistors to the coil is known conventionally as an H-bridge. The H-bridge controls the direction of the coil current Iw, by raising the voltage on one node (HY or HX), this voltage level is referred to as the high side voltage, and lowering the voltage on the other node, this voltage level is referred to as the low side voltage. The current direction can be changed by raising the low side voltage to the level required to become the high side voltage and lowering the high side voltage to level required for it to become the low side voltage. Changing the direction of the coil current Iw changes the magnetic flux through the coil.
One limitation of this conventional voltage source based write coil driver 20 is the maximum differential transient voltage generated at nodes HX and HY, during a current reversal. This limitation can be appreciated in conventional circuit 20 shown in FIG. 2, whereby the voltage of the low side voltage node can be momentarily pulled down, such as node HX, to near the second voltage rail Vee using transistor Q1. However, during this same time there is no circuit to pull up the high side voltage node HY. Thus, the voltage at each node can go no higher than that expressed by the following equation:
Vnode=Vccxe2x88x92(Vbe+RSxc3x97Iw).
Assuming the coil current Iw has a nominal current of about 40 milliamps. It is controlled by signals QL, {overscore (QL)}, QH, QH, {overscore (QH)}, QB and {overscore (QB)}. FIG. 8 shows the voltage levels for all four signals for a current transition with current initially flowing through the coil from node HX to node HY and then reversing and flowing from node HY into node HX. These signals are not complements in the strict sense but they are in the sense that the voltage levels for a current transition in the opposite direction can be accurately represented by the same wave forms but swapping the names, QL for {overscore (QL)}, QH for {overscore (QH)}, and QB for {overscore (QB)}. As is shown in FIG. 8 during current reversal of the coil, transistors Q1 or Q2 are temporarily turned on using signals QB and {overscore (QB)}to xe2x80x9cboostxe2x80x9d normal current and increase the differential voltage, decreasing the rise time and fall time commonly referred to as (TRTF). It can be appreciated that when voltage level of HX is transitioned from the high side voltage level to the low side voltage level, commonly referred to as pulled down, transistor Q1 will momentarily turn on and bypass half of the matched impedance resistors RS. During this xe2x80x9cboost,xe2x80x9d however, HY is transitioning from the low side voltage level to the high side voltage level or pulled up. The series resistor RS connected to node HY is still in the current path. Thus, the above equation represents the maximum pull up voltage on node HY when the node HX is pulled down close to Vee. Conversely if HX was being pulled up it could only achieve the maximum voltage level of the above equation where Q2 would momentarily turn on and bypass half of the matched impedance resistors RS allowing HY to be pulled down close to Vee. Conventionally, the maximum pull up voltage at either node is approximately 2.7 volts below the positive voltage rail Vcc.
There is desired an improved write coil driver circuit that improves the maximum differential transient voltage across nodes HX and HY during a current reversal of the coil, decreasing TRTF, and reducing the width of the overshoot to achieve faster write data rates. This can be achieved by providing a high side voltage boost that bypasses the matched impedance resistors RS during the pull up. This boost is realized by including a xe2x80x9chigh sidexe2x80x9d drive using large PMOS FETs one connected between HX and VCC and the other connected between HY and VCC, 32 in FIG. 3.
The present invention achieves technical advantages as an enhanced voltage drive circuit for a HDD write driver by providing circuitry that pulls up the voltage at one end of the coil to approximately Vcc effectively bypassing the associated series resistor RS. During a current reversal, the high side boost devices preferably PMOS FETs, significantly improve the maximum differential transient voltage at nodes HX and HY. This increased voltage decreases the TRTF and has the potential to greatly increase the overshoot with out increasing the overshoot pulse width or the settling time thereby obtaining a faster data rate.